I never really saw the appeal of an Ash/Lillie ship. I can see where people are going when they see it as likeable but for me, most of Lillie's interactions with Ash are pretty much her being grateful for everything he's done to her. Justapoxing her with Serena, I'd say "gratidute" sums up Lillie's view on Ash.
Technically the way every female character have felt for ash was written from the very beginning of each Series, thats exactly why amour became so popular even before that series started, i remember that back in 2013 when the finals episodes for BW were airing (thank god that series was awful hahaha) and they were promotion XY series in TV spots, literally the first spot they gave us where serena appeared was one where she had a flashback of seeing ash when they were kids and that detonated a bomb in the fandom speculating what kind of relationship she and him would have (obviously the staff for that season did it in porpuse to attract people).
The same can be said about any other generation, Ash and misty had a bros relationship (being misty the older one) however in America she had more attention because in the Dub the changed some dialogues to make it looks like if they liked each other.
With May and Ash the situation was the same but this time Ash was the older brother, with Ash and Dawn here it was a bit complicated, some people may look at this relationship as the one ash had with May however as the season progressed there were obvious hints that both could have ended in a relationship (even some characters thought that they were a couple i.e Lyra and Kenny), however nothing happened because they had to make ash lose so he would participate in the next season.
With Iris their relationship is not even as friends, at the most they are at good terms and thats it (anytime they see each other they complain).
And here is where we arrive to Serena, as i said the kind of relationship each pokegirl (except for Dawn´s) would have with ash was written since the beginning of their respective series, and thats why the expectations on XY(Z) last episode were so high, that episode would define if any ship was with ash was worth to support or not, and well with that kiss they proved us that indeed amourshipping was worth to support.
With mallow and lana they had a relationship with ash that was a mix as the one he had with iris and may, they respected him and were friends but thats it, however with lillie it was more like a mix of the relationship ash had with may and serena, he was the source of her inspiration (just like he represented that to may), but also some of her behavior could have been interpreted as if she felt something for him (just like what happened with serena), however it was more because he helped her overcome her fear for Pokemon than anything else, of course shes thankfull for that but thats what he represented for her, a source of inspiration to be brave and courage.
In any case, if Ash had actual posibilities to be in a relationship with any of these Pokegirls they would have had teased it from the very beginning of their respective series.
Thats why if Ash doesnt end up together with Serena its extremely unlikely that he will ever do it with someone else.
